Electric furnace.



T. B. ALLEN.

ELECTRIC FURNACE.

APPLICATION FILED APR. 24, 1912.

Patented Feb. 25, 1913.

WITNESSES UNITED STATldb THOMAS B. ALLEN, OF NIAGARA FALLS, NEW YORK,ASSIGNOR TO THE CARBORUNDUM COMFANY, BF NIAGARA FALLS, NEW YORK, ACORPORATION OF PENNSYLVANIA.

ELECTRIC renames.

To all whom it may concern Be it known that I, THOMAS E. ALLEN, aresident of Niagara Falls, in the county of Niagara and State of NewYork, have invented a new and useful Improvement in Electric Furnaces,of which the following is a full, clear, and exact description,reference being had to the accompanying drawings, forming part of thisspecification, in which Figure 1 is a vertical cross section showing oneform of furnace embodying my invention; and Fig. 2 is a horizontalsection of the same.

My invention relates to electric furnaces of the resistance type, andparticularly to the furnace employed for the conversion of shapedarticles into silicon carbid, such, for example, as the articlesobtained in the proccss described in United States Letters Patent No.992,698, granted to Frank J. Tone on May 16, 1911. In such processes,molded articles of silicon carbid and carbon are buried in the mixtureof silica and carbon and subjected to a temperature suflicient to formsilicon-containing vapors, which penetrate the molded articles andconvert them into solid articles of substantially pure silicon carbid.In such processes as heretofore practised, the majority of the energysupplied was consumed in converting the enveloping mixture of silica andcarbon into silicon carbid, a relatively small amount of the energybeing available for the desired Work of silicidizing the articles to betreated. In other words, such furnaces have had a very small capacity inproportion to the power used. Furthermore, uniform radia tion from theresistance core into the article being treated was not obtained and thearticles were subjected to displacement and other difficulties.

One of the chief uses of the silicidized ma terial or silicon carbid inquestion is for electrical resistance units. It has many desirablephysical qualities for this purpose, such as high specific resistanceand high resistance to chemical reagents. On account of its electricalresistance, however, it is not possible to form more than acomparatively short length of resistance; therefore, to consume asufficient quantity of electrical power to obtain the desiredtemperature, it is necessary to use the units in multiple. If this isdone, it evidently is important that the Specification of LettersPatent.

Application filed April 24, 1912.

Patented Feb. 25,1913.

Serial No. 692,825.

resistivity of the various units should be practically identical,especially, as the material has a negative temperature coefiicient. Afurther difficulty arises in that the coeflicient of resistance varieswith the com position of the converted article, that is,

amorphous silicon carbid is given in the latest publications as 1520 C.and that of crystalline silicon carbid as 1820 C. and that of thedecomposition of crystalline silicon carbid as 2220 C. There is,therefore, a drop of from 500 to 700 C. in the conversion zone of a fewinches in thickness, and this makes most difficult the obtaining of auniform product. It is also of primary importance to have the furnaceconditions and methods of heating under absolute control, since varyingconditions give w dely varying physical properties. Some form ofconversion zone which Will give better control over the temperature andvapor cond tions and method of heating has been necew sary, and I havediscovered that a much more uniform condition of temperature and abetter permeation of the silicon vapors can be obtained in thesilicidizing or conversion zones when an inclosed chamber is used, whichis practically shut off from the outside atmosphere and communicatesonly with the atmosphere of the furnace.

In the drawings, 2 represents the base; 3, 3 the head walls, and 4, 4cthe side walls of a furnace, these being of suitable refractory materialforming a box-like chamber to con.- tain the cores, conversion Zones andenveloping mixture.

5 designates the electrode terminals projecting through the head wallsand connected to the source of electrical power.

6, G are carbon resistance cores, which are preferably of granularcarbon, though other resistance material may be employed, and cores ofblock form or other solid shape may be used. The resistor is soarranged. pref erably by elongating it vertically, that it will give themost uniform possible radia-. tion of heat into the walls in theintermediate conversion zone. This conversion chamber, shown at 7, ispreferably formed of refractory material, such as bricks or tile ofsilicon carbid. The walls facing the cores are preferably perforatedwith holes so that preferably entirely surrounds the cores andconversion chamber, except that the latter may be supported from thebase of the furnace. The proportio-ning of the cores and the dimensionsof the furnace will follow the practice in the well known granular-core.

silicon carbid furnace, the principles of which are well known to thoseskilled in the art.

The furnace is operated in the same manner as a silicon-carbid furnace,and the run is preferably continued until the mixture between the coresand silicidizing zones is converted into silicon carbid. Thesilicidizing effect may be varied Within considerable limits. After theoperation has continued a sufficient time to give the desired degree ofconversion to the articles which have been placed in the conversionchamber, the furnace is shut down to allow them to cool, the

' chamber is uncovered, and the articles unloaded.

It is important that the surrounding mixture shown at 10 should besufliciently porous to permit free circulation of siliconcontainingvapors, and in place of using sawdust 'or similar material for suchpurpose, it is possible to use the silica and carbon in coarser lumps ora mixture thereof in briquet form. I have also found in certain casesthat the addition of silicon carbid to the silica carbon mixture willgive im v proved results. In such cases, this reduces the shrinkage andsettlement of the mixture, aids in the conduction of heat from the cores.to. the silicidizing zones, and has certain other advantages.

The advantages of my invention result from the use of the conversionchamber, which is substantially closed in from the outer atmosphere andis open only to the atmosphere of the furnace. By the use of suchfurnace, I am enabled to obtain much more uniform results in theconverted articles, thus making them of much higher value.

and widening their range of usefulness.

Many changes may be made in the form and arrangement of the furnace, thecharacter of the charge, cores, conversion chamber, etc., withoutdeparting from my invention.

I claim:

1. An electrical silicidizing furnace. containing a silicidizing chamberin communicat-ion only with the atmosphere of the furnace means forobtaining a circulation ofsilicon-containing vapors in said chamber, andresistors arranged to heat said chamber.

2. An electrical silicidizing furnace, containing a silicidizing chamberin communication only with the atmosphere of the furnace, resistorsarranged to heat said chamber, and a porous mixture containing silicaand carbon surrounding and separating said chamber and resistors.

3. An electrical silicidizing furnace, having a plurality of resistors,a silicidizing chamber between the resistors and open only to thefurnace atmosphere, and a porous mixture containing silicon and carbonsurrounding the resistors and the interposed conversion chamber.

4:. An electrical silicidizing furnace having a porous charge, resistorsembedded in the charge, and a conversion chamber between the resistorsarranged to allow vapors to enter its side walls adjacent to the cores.

5. An electrical silicidizing furnace, having a porous charge, a closedconversion chamber open only to the furnace atmosphere, and a resistorembedded in the charge containing silica and carbon and elongatedsubstantially parallel to the conversion chamber.

In testimony whereof, I have hereunto set my hand. THOS. B. ALLEN.

Witnesses:

MARTIN HAWKE,

F. EASLOE.

